Two-way radio voice communications systems have become extremely popular in recent years, and so much so that the usual prior art voice-communication systems are no longer adequate. Digital and tone signalling equipment are presently in widespread use to provide selective calling in present-day radio voice communication systems, especially in circumstances where the radio system is interconnected to a telephone exchange. The "Touch-Tone" multi-frequency system developed by American Telephone and Telegraph Company for its telephone switching networks has proven to be adaptable to such radio voice-communication systems.
The usual multi-frequency encoder is designed so that when a particular key or pushbutton on a keyboard is depressed, a pair of signalling tones is produced, with each tone being carefully selected to avoid the principal harmonics normally created in speech and music. Multi-frequency signalling has been adapted to radio communication, and has been used satisfactorily in prior art two-way radio voice-communication systems.
The usual prior art multi-frequency encoder unit for mobile radio transmitters includes a keyboard and an encoder generator circuit connected to the keyboard. The encoder generator circuit generates the two distinct tones whenever a particular key on the keyboard is depressed, and these tones serve to identify the particular key. The dual tones are transmitted by the mobile transmitter to the receivers in the system in which they are decoded. In this way, a call may be placed to a selected receiver by depressing a number of keys on the keyboard in a particular sequence corresponding to the selected code for the particular receiver.
The keyboard-encoder unit of the present invention includes a relay which activates the transmitter only when the keys on the keyboard are operated, so that the transmitter is automatically turned off whenever the keys are released obviating any need for a separate main switch.